1. Field of the Invention
The description set forth herein generally relates to display members and methods of manufacturing them. More particularly, the description relates to electronic paper-type displays having enhanced contrast and brightness properties.
2. Description of Related Art
Display technologies based on encapsulation of electrophoretic particles, multichromal beads and liquid crystals have many potential applications in fields such as digital document media, for example, electronic paper. High brightness and high contrast are two of the main performance requirements for digital document media applications.
In particular, bichromal displays have numerous advantages over conventional electrically addressable visual displays, such as LCD and CRT displays. In particular, they are suitable for viewing in ambient light, retain an image indefinitely in the absence of an applied electric field, and can be made lightweight, flexible, foldable, and with many other familiar and useful characteristics of ordinary writing paper. Thus, at least in principle, they are suitable both for display applications and for so-called electric paper or interactive paper applications, in which they serve as an electrically addressable, reuseable (and thus environmentally friendly) substitute for ordinary paper. For further advantages of the bichromal display, see U.S. Pat. No. 5,389,945, which is herein incorporated by reference.
Although bichromal displays promise to offer many of the advantages of ordinary paper together with the advantages of electrically addressable displays, the bichromal displays can be improved. For example, some displays do not look as good as paper. In particular, they may not have the high white reflectance of paper (typically 85% diffuse reflectance for white paper). Consequently, they do not have the high brightness and contrast characteristics of ordinary white paper.
Typically a way to improve the white reflectance of a bichromal display is to make the display from a thick arrangement of bichromal balls. It is thought that the thicker the arrangement of balls, the better the reflectance and the brighter the appearance of the display. The analogy here is to ordinary paint: other things being equal, a thicker coat of white paint reflects more incident light than a thinner coat of paint, and therefore appears brighter and whiter than the thinner coat. By analogy, it is expected that a thick arrangement of bichromal balls will tend to reflect more incident light than a thinner arrangement. In particular, the white faces of bichromal balls located at some distance below the viewing surface of the display are expected to reflect any light that is not reflected by balls located nearest the surface.
It is also thought that to achieve high resolution in a bichromal display, the cavities in which the balls rotate should be packed as closely together as possible. However, it is conventionally supposed that the size of the balls within the cavities is of no consequence insofar as display reflectance is concerned. That is because in a display having a thick arrangement of bichromal balls, the balls located farther from the viewing surface of the bichromal display will “fill in the gaps” between bichromal balls located nearer the viewing surface. In other words, so long as the two-dimensional projection of the balls at all distances from the viewing surface onto the viewing surface substantially covers the viewing surface, a high-quality display will be obtained.
However, the display manufacturing techniques described above may result in a thick display, which has certain drawbacks. Notably, a thicker display, requires a higher drive voltage. Nevertheless, virtually all known bichromal displays are made with thick arrangements of bichromal balls (such as sheets of bichromal balls wherein the sheets are several ball diameters thick), because this is thought to be necessary in order to produce displays of adequate brightness.
Additionally, current multichromal display devices are produced by the “swollen sheet” method. In this method bare bichromal beads, randomly mixed and dispersed in a silicone elastomeric sheet, are rendered rotatable by swelling the elastomer in silicone oil. Pockets of oil form around each bead, and the beads detach from the elastomer-bead interface. However this method has many shortcomings including a relatively low white reflectance which yields low brightness, low contrast and a high switching voltage threshold, which results from the thicker multi-layered structure of the randomly dispersed beads.
Accordingly, a need exists for a display device that is capable of achieving high optical contrast, high brightness and low switching voltage and a method of manufacturing thereof.